Mach-Zehnder interferometers are commonly used in various applications including optical telecommunication applications. Mach-Zehnder interferometers are optical filters, which may be employed in optical-signal analysis systems, optical-communication link-characterization systems or in transceiver devices, for example. Mach-Zehnder interferometers, in combination with phase modulators, are also used at the basis of Mach-Zehnder modulators. Other possible uses include Differential Phase Shift Keying (DPSK) receivers, power combiners, interleavers, optical switches, etc., which find applications in Code Division Multiple Access (CDMA) communications, secure communications and Optical Coherence Tomography (OCT), for example.
A Mach-Zehnder interferometer is an interferometer for which input light is split onto two distinct light paths having different path lengths and then recombined to interfere. The path-length difference may be fixed or varied using, for example, a phase modulator to provide a Mach-Zehnder modulator. When a Mach-Zehnder modulator device comprises both a fixed delay (i.e. much greater than the wavelength λ of the input light under normal operation) and variable delay (i.e. of the order of λ), the device is often termed as an “unbalanced Mach-Zehnder modulator”. The few commercial vendors that offer unbalanced Mach-Zehnder modulators based on integrated optics generally offer only a particular value for the path-length difference, which often does not correspond to the value needed by the user. There is no practical way to change this value once manufactured. The fabrication of an integrated-optic Mach-Zehnder modulator having a custom path-length difference value is an expensive process, involving the fabrication of a custom mask, and hence, when manufactured in small quantities, leads to very high unit costs.
It could be envisaged that the Mach-Zehnder interferometers be fabricated using optical fiber rather than integrated-optic waveguides for the path lengths, wherein the optical fiber could be polarization-maintaining in order to minimize deleterious effects arising from acoustic and mechanical perturbations. However, it is generally impractical to construct interferometers having path differences of the order of a few millimeters or less, since it is almost impossible to reliably and precisely splice (or otherwise mate) fiber lengths having such small controlled length differences. A variable or adjustable optical delay line could be employed to achieve a particular desired path length difference, but such a device is generally expensive and bulky, and hence is often unacceptable. Generally speaking, an interferometer constructed with two optical fibers may be unduly sensitive to temperature changes because a differential temperature change on the two macroscopically large (generally >20 cm) distinct optical-fiber paths changes the effective path-length difference.